1. Field of the Invention
This invention relates to long life batteries utilizing recombinant cells and to such cells.
As used herein the word xe2x80x9ccellxe2x80x9d, including plurals and variants thereof, denotes a single electrochemical unit having at least one positive plate, at least one negative plate and separator material between those plates, all within a thermoplastic housing and nominally providing 2.0 volts potential.
As used herein the word xe2x80x9cbatteryxe2x80x9d, including plurals and variants thereof, denotes a plurality of electrically connected cells providing a specified voltage and a specified current over a specified time.
2. Description of the Prior Art and its Problems
Recombinant lead-acid cells and batteries are known, being sold by a variety of manufacturers in the United States and elsewhere. One well-known supplier of recombinant lead-acid batteries is C and D Charter Power Systems, Inc., which sells recombinant lead-acid batteries under the trademark xe2x80x9cLiberty Series.xe2x80x9d
Recombinant lead-acid cells are disclosed in U.S. Pat. No. 3,862,861.
A continuing problem faced by manufacturers of lead-acid cells in endeavoring to provide long life batteries utilizing such lead-acid cells, especially recombinant lead-acid cells, is inherent growth of the positive plate due to corrosion and oxidation of the lead or lead alloy grid to form lead dioxide. Because the specific volume of lead dioxide is about 21% greater than that of metallic lead, as the lead dioxide corrosion product forms, the grid grows due to built up stress. This leads to gradual loss of physical contact and electrical continuity between the grid and active material pasted on the grid and may eventually cause the grid to fracture. Loss of electrical continuity may result in failure of the cell in which the grid is located.
Another common cause of failure of such cells (which is also rooted in the plate growth phenomenon) is shorting. This occurs when positive and negative plates contact, due to stresses created within the cell, as the positive plates grow.
Positive plate growth has been known for years, being reported in xe2x80x9cPositive Grid Design Principlesxe2x80x9d published in The Bell System Technical Journal, September 1970. While the phenomenon has been long known, growth of such plates and difficulties resulting therefrom is a continuing problem in lead-acid cells intended for use in long life batteries.
An additional problem sometimes encountered in recombinant cells intended for long service life is the tendency of dendrites to grow from the negative plates, especially if free electrolyte is present in a cell. The likelihood of dendrite growth is enhanced if free electrolyte is present. Free electrolyte sometimes forms in a cell during operation. Any free electrolyte collects at the bottom of the cell and hence the likelihood of dendrite growth is greatest at the cell bottom. If a dendrite grows from a negative plate to a positive plate, the plates short, damaging and possibly disabling the cell.
Another problem in recombinant cells intended for long service life is maintenance of close contact between the positive and negative plates and the microporous separator material between those plates. Close contact is important because the electrolyte is provided in only a starved amount and only part of the starved amount of electrolyte resides within the separator material. If good contact between the plates and the separator material is not maintained, the recombinant cell will not function properly.
An example of the long life battery of the general type to which this invention relates is available from the GNB division of Pacific-Dunlap, Ltd. under the trademark xe2x80x9cAbsolyte.xe2x80x9d
While the Absolyte system has achieved some commercial acceptance, it does not provide for external application of compression to its recombinant lead-acid cells. Such compression is desirable to assure maintenance of good plate-separator contact so that the electrolyte properly interacts with the plates.
Another problem in long life batteries of the general type to which this invention relates is the difficulty of replacing a cell upon failure of one of the cells in the battery. In the Absolyte system, cell replacement is cumbersome.
Yet another problem in many lead-acid cells intended for long service life is failure of the seal between the cell jar and cover, especially during manufacture. Typically during manufacture a substantial vacuum must be drawn in the cell to effectuate electrolyte flow into and distribution within the cell in the required amount and manner. Drawing a vacuum in the cell creates a substantial force on the cell jar as atmospheric pressure outside the cell presses on the jar and cover. Typically, the weakest point is the jar-cover joint or seal. Cells are prone to fail at these seals during manufacture when vacuum is drawn in the course of the electrolyte filling process.
Yet another problem in long life batteries is lack of adequate cooling for the multiple cells used to provide the required power. Typically, in the interest of saving space, cells are closely packed together without significant provision for active or passive cooling. As a result, overheating can be a problem.
In one of its aspects this invention provides a long life recombinant lead-acid battery defined by a group of recombinant lead-acid cells.
The cells may be grouped modularly in a plurality of vertically stacked interchangeable horizontal rows with facing surfaces of horizontally adjacent cells having vertically extending cooling channels formed therein. Cooling channels of the respective vertically stacked horizontal rows are substantially vertically aligned. Means are provided, preferably in the form of planar sheets, for maintaining the channels of the horizontally adjacent cells in separated disposition respecting one another.
When the cells are arranged in vertically stacked horizontal rows, plates supporting the cells preferably have holes which at least partially intersect the vertically extending channels, permitting convective air flow in a substantially vertical direction between the horizontally interchangeable adjacent cells which are arranged in vertically interchangeable rows.
In another aspect, the invention provides a long life battery including a plurality of recombinant lead-acid cells with means for applying and maintaining compressive force to pluralities of interleaved positive and negative plates, and separator material therebetween, within the cells. The force application means is preferably manually actuated and continuously applies force, preferably in a direction perpendicular to the plates. The force application means preferably includes vernier means for manually applying force selected from a continuum of available force values.
In another aspect this invention provides a long life recombinant lead-acid cell. The cell includes a case, a plurality of interleaved positive and negative lead metal alloy plates within the case, microporous separator material between respective positive and negative plates, and means for suspending the plates within the case spaced away from the case interior surfaces in the direction of plate growth without contact between positive and negative plates.
The plate suspension means aspect of the invention includes insulative, preferably planar, means for engaging the negative and positive plates and maintaining the negative and positive plates in spaced relation respecting both one another and the case in the direction of plate growth. The suspension means preferably engages the positive and negative plates at or close to respective ends of the plates and permits positive plate growth without positive plate/negative plate contact, which would produce a disabling short. The suspension means reduces risk of contact between the positive plate and the cell case, in the direction of plate growth, upon such plate growth. Such contact can increase internal stresses in the positive plate, eventually leading to plate and cell failure.
The cell advantageously lies horizontally. The plates advantageously are vertically disposed. The plate suspension system accommodates plate growth in the longitudinal direction, which is the direction of maximum plate growth, while centrally locating and supporting the plates. A portion of the plate suspension system guides a sandwiched plate-separator material assembly into the jar during cell manufacture and constrains the plate-separator material assembly from excessive movement once the assembly is in place.
Another aspect of this invention relates to plate wrapping. In this aspect of the invention, the plates are preferably wrapped with separator material substantially enveloping the plates, preventing them from shorting while permitting growth. The plates are advantageously wrapped in individual sheets. The sheets are folded about longitudinally extending edges of respective positive and negative plates. The plate wrapping aspect, together with the cell orientation aspect of the invention, reduces chances for shorts caused by dendrites from a negative plate contacting a positive plate.
In another aspect this invention provides a lead-acid cell having a stronger jar/cover seal. The cover overlaps the jar and preferably extends outwardly respecting the outer surface of the jar. A weldment of joined jar and cover material or cement connects the jar and cover substantially along the preferred optional portion of the cover contacting the jar and along the portion of the cover extending outwardly from the jar. The weldment if thermally produced is preferably homogeneous jar and cover material. The cover geometry at the position of cover engagement with the jar contributes to a larger and therefore higher strength weldment or cement bond which, in turn, contributes to the ability of the cell case, particularly the jar/cover seal portion of the case, to withstand high negative pressures during cell manufacture.
In a related aspect, this invention provides a cell case cover including an integral skirt extending generally transversely from the cover along an inner surface of the jar. The skirt reinforces the jar, in the area of jar-cover contact, to counter force resulting from pressure within the cell being lower than atmospheric. The skirt preferably substantially facingly contacts the jar inner surface and is of substantial thickness relative to its length, to provide structural reinforcement for the jar at the jar-cover joint or seal. The skirt makes the jar-cover joint or seal more resistant to fracture caused by pressure differentials between the cell interior and exterior.
In yet another aspect this invention provides an improved positive plate for lead-acid cells. The positive plate includes a grid frame having an outer periphery and at least one intermediate member extending between spaced apart portions of the grid periphery. The outer peripheral member of the grid is preferably of polygonal cross-section.
The grid preferably includes a plurality of elongated polygonal cross-section members extending between spaced portions of the peripheral member. The elongated members intersect within the outer peripheral member to define an open lattice. The lattice includes adjacent paste holding confinements offset from and communicating with one another transversely to the grid. These confinements hold paste to form the plate.
The intermediate member is preferably of polygonal cross-section and connects the elongated members at positions intermediate respective extremities thereof. The peripheral and intermediate polygonal cross-section members preferably have common length sides.
The polygon of the intermediate member preferably has at least two or more sides than polygons of the elongated members.
The polygonal shapes of the peripheral member, the intermediate member(s) and the elongated members strike an effective compromise among grid strength, grid growth, paste capacity and paste-grid adherence.
The intermediate member(s) are of substantially larger cross-section than the elongated members and preferably have substantially lower ratios of surface area to cross-sectional area. As a result, the intermediate members grow less than the elongated members as the lead oxidizes to lead dioxide.
In yet another aspect the invention provides an external cover or safety plate suspension system for cells, modules, batteries and other hazardous equipment. The cover plate suspension system permits the cover plates to be quickly positioned on or removed from the battery, or an individual module or a cell, without use of tools. The cover plate suspension system includes a bracket having a groove receiving the cover plate edge, with the groove having a mouth wider than thickness of the retained edge of the plate. The groove preferably includes bottom portions concavely convergingly curving respecting the groove mouth, from positions of separation greater than edge width to positions of separation less than safety plate edge width. The edges of the cover plates contact the curved bottom portions to provide an interference fit whereby the bracket releasably retains the cover plate.
In a yet further aspect of the invention, the lead-acid cell case may include a tubularly extruded thermoplastic circumferential jar with covers affixed to the ends of the extruded jar.